The Federal Aviation Administration (FAA) requires all passenger carrying aircraft over 30 seats be equipped with so-called “Mode S” transponders. Mode S transponders are capable of transmitting a number (e.g., 25) of formats of coded data. This coded data includes such information as a unique 24-bit binary address for each aircraft.
The aircraft registration number may be derived from this 24-bit binary address. The coded Mode-S data also includes such information as aircraft altitude and may be transmitted continuously throughout a flight at a minimum rate of 1 Hz (i.e., once per second). Coded Mode-S data may be collected passively without any connection to air traffic control equipment.
The FAA has endorsed the Aircraft Communications Addressing and Reporting System (ACARS) system, which uses various data link technologies including the VHF communication band, HF and SATCOM along with a ground station network to allow aircraft to transmit and receive messages of coded data. Many domestic and international carriers have equipped their aircraft with ACARS equipment.
ACARS equipment is capable of transmitting a number of types of coded data. ACARS currently uses frequency shift keying (FSK) as a modulation scheme, however, other modulation schemes including minimum shift keying (MSK) and time division multiple access (TDMA) are being evaluated for future improvement of ACARS. ACARS data includes such information as the aircraft registration number and airline flight identification number (flight number).
ACARS transmissions from a single aircraft may be sent at varying intervals from as little as no transmissions in a single flight to several transmissions per minute. ACARS transmissions may be collected passively without any connection to air traffic control equipment.
Triangulating on an aircraft's transponder signal may require decoding real-time transponder replies at several locations, time-stamping them and sending them to a central location for matching. Matching would attempt to pair up the transponder signals that had emanated from the same target.
An example of a Prior Art method for triangulating on an aircraft's transponder is disclosed in Wood, M., L., and Bush, R., W., Multilateration on Mode S and ATCRBS Signals at Atlanta's Hartsfield Airport, Lincoln Laboratory Project Report ATC-260, 8 Jan. 1998, incorporated herein by reference. In that method, triangulation on an aircraft's transponder relied on each remote sensor time-stamping all or most received transponder signals and passing them along to the central location for matching.
It was deemed necessary to do this since the remote sensor could not know which particular reply would be used by the central server for the matching process. This meant that a relatively high bandwidth communications medium was required between each remote sensor and the central server.
Such Prior Art methods used active interrogations to elicit the transponder replies, which allowed for some form of expectancy time for the replies. By scheduling interrogations the system estimated when replies might be received at each of the receivers and the system could then use windows in which to “listen” for replies. All replies received within these windows would then be time-stamped and then sent to the central server for matching.
This approach helped in some form to manage the required bandwidth on the link between the receiver and the central server. However, a relatively high bandwidth link is still required using this approach. Because of the practical bandwidth challenges in managing the link between the receivers and the central server it was generally thought in the Prior Art that using a completely passive approach for triangulation and multilateration would be impossible.
Multilateration and ASDI may be augmented with airline flight information available from an airlines flight reservation system. Dunsky et al, U.S. patent application Ser. No. 10/136,865, filed May 1, 2002 (Publication Number 2003/0009261 A1, published Jan. 9, 2003) entitled “Apparatus and method for providing live display of aircraft flight information”, incorporated herein by reference, describes the integration of Megadata's Passive Secondary Surveillance Radar (PASSUR) and airline flight information.
Multilateration may be used for a number of purposes. Flight tracks may be recorded and displayed for informational purposes for airlines, the general public, or for government agencies. Thus, an airline or consumers may be able to determine location of aircraft, or a government agency may be able to track aircraft for noise measurement purposes. Airports may be able to use such multilateration techniques to track landings and takeoffs and for billing landing fees. Multilateration may also be used to track ground vehicles or other targets.
Multilateration may be used to create a 2-D or 3-D track, depending upon application. In some applications, a 3-D track (including altitude) may not be required, such as for gross tracking of aircraft for an airline or the like. For other applications, such as noise measurement or landing fee tracking, a 3-D track (including altitude information) may be required.
When generating aircraft tracks from multilateration data, situations may occur in which multiple tracks are generated for the same aircraft. Due to the plethora of transponder types and formats (e.g., Mode A, Mode C, and Mode S), multiple transponder signals and signal types may be generated in response to interrogations from ground equipment or as the result of squawks and the like. A multilateration system may interpret these multiple signals as being from different aircraft, due to the different formatting of the signal types, and generate multiple tracks for the same aircraft. Thus, it remains a requirement in the art to detect such multiple tracking and combine track data into a single track for each aircraft.
In a multilateration target tracking system, not all sensors are able to supply a TOA (data time stamp) value for every single reply message of any target. Because of geographical placement of sensors and the statistic nature of TOA measurement noise, these diverse sets of sensor TOA combinations yield raw multilateration equation solutions that spread all over the place, even from one message to the next. In order to accurately track a target, it may be necessary to use different combinations of receivers to generate a complete target track. A technique is thus required to determine which combination of receivers to use in order to generate the most accurate possible target track.
Thus, it remains a requirement in the art to provide a multilateration system which can more accurately track aircraft or other vehicles by eliminating the inaccuracies noted above as well as other effects that affect accuracy of multilateration data as will be discussed in more detail below.